Number of Wires Through Joist Holes

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Old 08-31-08, 11:21 AM
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Number of Wires Through Joist Holes

My rewire is progressing, but another question . . . photos of the project are below to refer to.

I'm having to drill through a double joist/beam that splits my garage in order to run cables from the new panel location to the house via the attic. After I drilled a couple holes last night I thought it would be wise to find out how many I should create, wanting to limit them to as few as possible, but needing to allow for future expansion too.

How many cables can I safely take through these holes? I'll have only two 10 AWG circuits (dryer and A/C) and everything else will be 12 or 14. I've already drilled one 1" hole and made a 3/4 x 2 3/4 inch elongated hole. I expect to have a total of around 20-25 circuits that will need to pass through this beam. While there's plenty of space in my elongated hole for 6-8 cables, is this ok from a code/safety standpoint (heat generation, etc.). Or should I drill several more holes to spread everything out?

I've got two joist bays to work with and will be putting a running board up in there to staple the wires to. The running board will be attached to 2x4 nailers which will make drilling more holes in the future more difficult. That's why I'd like to get the drilling done now, even with future circuit possibilites in mind.



 
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Old 08-31-08, 11:41 AM
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There are several factors to consider: (1) maintaining the structural integrity of the joist, (2) having the cables fit through the hole comfortably, so that pulling them through does not damage them, and (3) not having so many cables through one hole as to create too much heat in the wires.

(1) There are many web sites that tell you where and how and how many and how large and how far apart to drill holes with minimal affect on structural integrity. You probably want to keep the holes out of the middle third of the span as much as possible. You want the holes dead center, top to bottom (as it appears ffrom the pictures that you have done). You don't want the holes too large or too close together. Search for some web sites with helpful diagrams.

(2) Don't stuff the cables through the holes tightly in a manner that will tear the sheathing. There are no firm guidelines here, but common sense applies.

(3) To avoid derating considerations (you don't really even need to know what I'm talking about here, but you can look it up if you want), do not put more than 9 current carrying conductors through one hole (more precisely, not through a series of holes). Don't count the grounding wires when doing this count. So you could put, e.g., four 12/2 cables through the same series of holes, but not five. If you do not have a series of holes (through adjacent joists), but rather the cables all go their separate ways after passing through the one hole, then this consideration doesn't really apply.
 
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Old 08-31-08, 01:01 PM
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So it sounds like you're saying that if this is the only hole the wires pass through, I'm good to go as long as I basically keep the number of wires to where they feel fairly free to move around in the hole. If they're tight, then it's too many.

This IS the only joist these cables will have to go through. I'll have them spaced out a bit on the running board I plan to install. For stapling the smaller stuff - 12 and 14 - is there a derating issue to even secure two cables under one staple on the running board? Or should all cables be totally separate from each other?

The structural integrity is definitely something I want to maintain, thus my wanting to use one hole for as many cables as possible while also maintaining good electrical design and safety.
 
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Old 08-31-08, 03:50 PM
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in the 2005 code, derating is applied to not only the cables going through the hole but the group that continues on to the next joist if that air space is fire or draft stopped. Than means, if the space is insulated, you must derate. The good news is you can derate from the 90 degree table as long as you do not use a result greater than the 60 degree limit.

It appears the room being wired is above a garage. The HUGE problem I see is there is no insulation or fire wall in place. In most areas, a fire wall is required between a garage and a living space.

That brings me back to my original intent. If you install insulation or prevent free air flow, you must derate all cables passing through the wood framing.
 
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Old 08-31-08, 04:17 PM
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The structural integrity is definitely something I want to maintain, thus my wanting to use one hole for as many cables as possible while also maintaining good electrical design and safety.
I believe you'll find, upon investigation, that a series of spaced apart smaller holes has less impact on structural integrity than fewer but larger holes. I definitely would not want to repeat that 3/4 by 2 3/4 hole.
 
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Old 08-31-08, 06:56 PM
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Originally Posted by nap View Post
It appears the room being wired is above a garage. The HUGE problem I see is there is no insulation or fire wall in place. In most areas, a fire wall is required between a garage and a living space.
Actually, the rewiring is for the whole house. The room above the garage is not living space. It's a bonus room only, not heated or cooled and not able to be legally made into a BR since the ceiling is too low over most of it. So I don't envision putting any kind of insulation into this joist bay.
 
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Old 08-31-08, 07:30 PM
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Originally Posted by John Nelson View Post
I believe you'll find, upon investigation, that a series of spaced apart smaller holes has less impact on structural integrity than fewer but larger holes. I definitely would not want to repeat that 3/4 by 2 3/4 hole.
Yep. Try not to drill holes like that anymore. Instead, drill a 3/4 hole, move over a few inches, drill one more 3/4 and so on. Also, drilling dead center of a joist is probaby the worst way to structurally weaken it. You've just efectively reduced that 2x6 to a 2X2{3} in that area.. The further away from the support wall your hole is, the worse it gets.

Electrical inspectors usually like to see you drill dead center but you'll piss off a structural inspector by doing that. The most logical compromise is to drill in the top 3rd or the bottom 3rd of a joist, but no closer than 1 1/4" from the bottom or top.

And as John has already said, the 3rd rule also applies length wise. Never drill in the middle 3rd of a joist. If it's overkill 50's and 60's framing, then an electrical inspector will usually let it slide, but best not to push your luck. If he woke up on the wrong side of the bed, it won't matter what size your joists are, he'll fail you. Then you've got to reroute all your home runs along with scabbing over all the holes you drilled. Stay with the first or last 3rd of the joist.
 

Last edited by madpenguin; 08-31-08 at 07:46 PM.
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Old 08-31-08, 09:49 PM
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Also, drilling dead center of a joist is probaby the worst way to structurally weaken it.
I respectfully disagree...........
 
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Old 08-31-08, 09:52 PM
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I'll recant. Due to opposite forces imparted on the top and bottom of the joists, you guys may be right.

That's what I like about being here. Learning better and more correct ways to do something. A few years ago, a structural engineer told me I shouldn't be drilling dead center and I took his word for it.
 
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Old 08-31-08, 09:59 PM
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Madpenguin.,,

I will disagree with one part you saying about drilling dead in centre of the joint useally it don't weaken at all if done right due the centre section is not compressing or tenstioing { push or pull } but only few area where the drilling is complety hands off is enginereed wood beams or trusses and high load factor wood Ibeams unless it is specifed by manufacter give you a instruction where to bore it.

I useally don't have much issue unless someone mess up the structure interitgy. { sp }

Merci,Marc
 
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Old 08-31-08, 10:05 PM
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Originally Posted by french277V View Post
...but only few area where the drilling is complety hands off is enginereed wood beams or trusses and high load factor wood Ibeams
That's good you brought that up. Easy to forget if your not around microlam's and specialty support beams that often.

Doesn't that also apply to blocking that spans the inside of joists to keep them from moving/twisting?
 
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Old 08-31-08, 10:12 PM
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I see no problem drilling blocking.
 
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Old 08-31-08, 10:12 PM
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Originally Posted by madpenguin View Post
That's good you brought that up. Easy to forget if your not around microlam's and specialty support beams that often.

Doesn't that also apply to blocking that spans the inside of joists to keep them from moving/twisting?
Not a problem.

I just want to make sure other reader to remember the microlam and engineered beams are very tricky to deal if not done right.

For the blocking materals it pretty much depending on which type of blocking and materal itself.


Merci,Marc
 
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Old 08-31-08, 10:24 PM
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Thanks guys. I was self taught for the first 4 years or so and I'm finding I have alot of word-of-mouth information still filled away as fact... I'm sure it'll all get sorted by the time I'm Journeyman.

I vaguely remember someone telling me not to drill blocking but I do it anyway if I have to, otherwise I'll take a different route.

Sorry Cakins if I've derailed your post at all.
 
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